The present invention relates to curable systems such as curable adhesive systems and is more particularly concerned with curable polymer containing systems having a relatively long pot life which comprise at least one curable epoxy resin and at least one imidazole type curing agent for the epoxy resin. Curable polymer containing systems find widespread utility as the applied starting material for producing coatings, fillings, linings, laminates, cores, encapsulants, adhesives and like on a variety of substrates.
The term "pot life" as used herein may be defined as the period of time a curable system is generally fit for its intended purpose under normal environmental conditions without special precautions. For most systems, these normal conditions are room temperature, humidity, etc. The pot life of a curable adhesive system is usually the period of time until gelation or hardening occurs, thereby making the system difficult or impossible to apply.
In a preferred embodiment of the present invention, the curable systems are adhesive type curable systems adaptable for joining two or more structures together in a semi-permanent or permanent manner. While the present invention will be primarily discussed hereinafter with reference to curable adhesive type systems, it should be understood that the invention is not thereby so limited.
In a continuous type manufacturing operation where articles are made by joining two or more structures with an adhesive bond formed from a curable adhesive system, it is generally preferably to utilize a curable adhesive system that will remain in the uncured state for an almost indefinite period before being applied to the structures to be joined, that is, have an almost indefinite pot life, will cure in a relatively short period of time without the application of harmful amounts of heat, and will still provide a strong adhesive bond.
One particular type of curable adhesive system which has been in widespread use is a system containing an epoxy resin alone or in combination with other type resins. Epoxy resins for these systems are polyethers generally composed of intermediate diepoxy compounds formed as the condensation product of epichlorohydrin and aromatic diols such as bisphenol A. These intermediate compounds may then be cured by a further cross-linking polymerization. The intermediate diepoxy compounds are usually soluble in organic type solvents which thereby facilitates application of these intermediates to various substrates. The term epoxy resin as used in this disclosure includes both the intermediate form and the final cross-linked form of the resin.
In many epoxy resin containing curable adhesive systems, a curing or cross-linking agent is included in the system so as to promote the rapid curing of the system once it has been applied to a substrate. Known curing agents for epoxy resins include imidazole compounds and various tertiary amines and amides such as dicyandiamide triethylene-tetramine. Other known curing agents include boron trifluoride and compounds having a reactive hydrogen atom such as organic acids, alcohols, mercaptans and primary and secondary amines.
However, the advantages obtained by the ability of the curing agent to promote curing of an adhesive system in a relatively short period of time must generally be balanced against the decreased pot life of the adhesive system due to presence of the curing agent unless the curing agent can in some fashion be inhibited or suppressed up to the time of application of the system. In many situations, the use of a particular curing agent in epoxy type adhesive systems has been thought to be generally precluded because of the relatively short pot life of the curable adhesive system.
For example, imidazole compounds have been known as effective catalytic curing agents for epoxy resins (see Imidazole Catalysis in the Curing of Epoxy Resins, A. Farkas and P. F. Strohm, Journal of Applied Polymers, Sci. Vol. 12 pp. 159-168 (1968)). The use of imidazole compounds as curing agents results in epoxy resins having excellent physical properties. However, the use imidizole type curing agents for epoxy resin containing adhesive systems has been limited as such systems have a very short pot life and thus their usefulness in continuous type manufacturing operations has been generally precluded.